1. Field of the Invention
The present invention relates to disk drives, and more particularly, to a driver and method for controlling a voice coil motor in a disk drive.
2. Discussion of the Related Art
Disk drive 10 includes head 12, disk 14, spindle motor 16, preamplifier 18, channel 20, microprocessor 22, digital-to-analog converter (DAC) 24, driver 26 and actuator assembly 28 that includes voice coil motor (VCM) 30 and actuator arm 32. Furthermore, VCM 30 includes coil 34 and permanent magnet 36.
Head 12 is a transducer that reads data from and writes data to disk 14. Head 12 is attached to or formed integrally with a slider. Disk 14 is a magnetic storage medium that stores data in concentric tracks. Spindle motor 16 rotates disk 14 so that head 12 is supported by a cushion of air (air bearing) at a flying height in close proximity to disk 14.
Preamplifier 18 amplifies analog read signals from head 12 and passes the read signals to channel 20, and channel 20 demodulates the read signals and sends digital signals to microprocessor 22. Microprocessor 22 sends a digital command signal to DAC 24, which transforms the digital command signal into an analog command signal, and driver 26 receives the analog command signal and sends a coil current to VCM 30.
VCM 30 is coupled to actuator arm 32, which is a suspension that supports head 12. VCM 30 rotates actuator arm 32 about a pivot point to move head 12 radially across disk 14 to selected tracks during seek operations, and maintains head 12 above selected tracks during track following operations.
VCM 30 is a fast response, direct current, pure torque motor that includes top and bottom plates (not shown) and coil 34 and permanent magnet 36 therebetween. The coil current passes through coil 34 to generate a magnetic field that interacts with the magnetic field of permanent magnet 36 to create torque that rotates actuator arm 32 and positions head 12. Coil 34 is a stacked coil with two coils stacked relative to one another. When the coil current passes through the coils in the same direction the coils generate forces in opposite rotational directions that cancel each other and no torque is generated, and when the coil current passes through the coils in opposite directions the coils generate forces in the same rotational direction that supplement one another and torque is generated. In addition, mechanical disturbance forces are balanced so that the coil current puts electrical energy into coil 34 that creates desired motion without wasting moment (pure torque).
Disk drive 10 receives read and write commands from a host computer (not shown), and in response, performs read and write operations in which head 12 accesses different tracks on disk 14. The read and write operations include servo operations which include seek and track following operations. During a servo operation, microprocessor 22 receives servo position information from head 12, implements a servo control program by executing an estimator control loop program, and commands driver 26 to send a coil current to VCM 30 to accurately position head 12 over the selected track in as short a time as possible to enable the data transfer between head 12 and disk 14.
Disk drive 10 increases its storage capacity by reducing the flying height of head 12 and by reducing the track spacing on disk 14. Reduced flying height increases the bits-per inch (BPI) on disk 14, and reduced track spacing increases the tracks-per-inch (TPI) on disk 14. However, actuator assembly 28 is a non-rigid structure that exhibits mild vibration at resonant frequencies during the seek and track following operations. As a result, actuator assembly 28 creates mechanical disturbance that can degrade the performance of disk drive 10 as the flying height and the track spacing are reduced. For instance, at low flying height the mechanical disturbance can cause head 12 to contact disk 14, thereby damaging head 12 as it sticks to varnish on disk 14 and ruining data at the contact point on disk 14. Likewise, at high TPI the mechanical disturbance can limit the servo bandwidth due to poor frequency response at the lowest resonant frequencies.
Actuator assemblies have been designed with secondary motors that position the head relative to the disk and reduce the mechanical disturbance. However, dual-stage actuators require more space in the disk drive.
There is therefore a need for improved position control of a head in a disk drive.